<p>Stainless steel is widely used in engineering and industrial applications, particularly in the aerospace, marine, automotive, and petrochemical industries, due to its excellent mechanical properties and cost-effectiveness compared to other materials. Additive manufacturing (AM) has emerged as a promising technique for fabricating complex geometries that are difficult to achieve through conventional manufacturing methods. However, AM-fabricated metals often suffer from major drawbacks such as porosity, anisotropic microstructures, and tensile residual stresses, which adversely affect fatigue performance, mechanical strength, and corrosion resistance. Friction stir processing (FSP), a solid-state severe plastic deformation technique, has recently gained attention as an effective post-processing method to overcome these limitations. This article presents a comprehensive overview of FSP’s impact on additive-manufactured stainless steels, focusing on how tool geometry, tool material, rotational speed, advancing speed, etc., refine grains, eliminate defects, and homogenize microstructure. Moreover, a comparative perspective across different AM techniques is also presented to highlight the unique challenges and opportunities that arise when combined with FSP.</p>